An Investigation into leaf surface temperature

Students’ Sheet

Introduction

“What happens to the surface temperature of leaves if petroleum jelly is applied to their lower surfaces?”

Your challenge is to plan and carry out an investigation that will producequantitative datathat will allow you to answer this question.

In this investigation, you will be using a portable infrared thermometer to make measurements of leaf surface temperature.At a distance of 1cm from the object being measured (the leaf surface), the infrared thermometer detects the infra-red radiation emitted from a circular spot with an area of 1cm2 on the object’s surface.

The leaves that you use must remain attached to the plants throughout the experiment.

Instructions

Your first challenge is to plan your investigation. You will measure the temperature of the upper side of each leaf before and after smearing the lower surface of the leaf with petroleum jelly.

Your plan must be sufficiently detailed to allow you to answer the following questions:

  • How many leaves should you test?
  • Will you use leaves from only one species of plant?
  • Will all the leaves you use be on the same plant?
  • Will all the leaves be approximately the same size?
  • Will all the leaves be at the same height on the plant?
  • Will all the leaves be facing the same direction?
  • How far away from the leaf will the infrared thermometer be held?
  • Will you measure the temperature from the same point on the leaf each time?
  • At what angle will the infrared thermometer be held to the leaf?
  • How long will the thermometer be pointed at the leaf for?
  • How long should we wait before we measure the temperature of the leaf after applying the Vaseline to the underside?
  • The investigation uses paired data i.e. the surface temperature of each leaf is measured first without Vaseline on the underside and then re-measured with Vaseline on the underside. How does this method improve the validity of the data?

Remember to include an assessment of the risks your investigation will poseto

  • yourself and others,
  • the environment and its wildlife

and the ethical implications of your investigation.

Questions

  1. Which plant species did you use?
  1. Record your data in a table with the following column headings:

Leaf number / Leaf surface temperature
before application of petroleum jelly to lower leaf surface/°C / Leaf surface temperature
after application of petroleum jelly
to lower leaf surface/°C / Difference in
temperature/°C
  1. Calculate mean values, standard deviation and standard error and 95% confidence limits for columns two and three in the table (Leaf surface temperature before application of petroleum jelly to lower leaf surface/°C and Leaf surface temperature after application of petroleum jelly to lower leaf surface/°C). Display your data in an appropriate graph or chart.
  1. Is there a significant difference between the mean leaf surface temperature without Vaseline and with Vaselineon the underside?
  1. If you think there is a significant difference between the two means in question 4, what do you think is the cause of this difference?
  1. Suggest a hypothesis that links your answer to question 5 to stomata and transpiration.
  1. Plants have different densities of stomata on their upper and lower epidermis.

(i)Outline how you could use the infrared thermometer to investigate differences in the stomatal density in the upper and lower epidermis of leaves.

(ii)What assumption are you making about the evaporation of water from the leaf in 7(i)?

(iii)How could you test any conclusions that you drew in 7(i)?

  1. This investigation has allowed you to see how science works. The following flow chart summarises how science worked in this investigation. Match up the description on the right with the correct step on the left.

Step / Description
  1. Observations
/ The leaf heating effect observed when the stomata are blocked will be greatest when the density of stomata in the epidermis is high
  1. Hypothesis
/ Do results and conclusions support the original hypothesis?
Yes-hypothesis is strengthened.
No-modify hypothesis in the light of the results of the experiment
  1. Predictions
/ Comparing the effect on leaf surface temperature of applying petroleum jelly to the lower epidermis (where there are lots of stomata) with the effect of applying petroleum jelly to the upper epidermis (where there are fewer stomata).
Investigating the stomatal density on the upper and lower epidermis of leaves
  1. Experiments
/ Evaporation of water from the stomata cools the leaf surface
  1. Analysis of results and drawing conclusions
/ There is a significant difference between the mean leaf surface temperature without Vaseline on the underside and with Vaseline on the underside

Science & Plants for Schools:

Investigating Leaf Temperature

This document may be photocopied for educational use in any institution taking part in the SAPS programme.

It may not be photocopied for any other purpose. Revised 2010.